I'd be down... I got in a Co-op match with a random person (super rare to find someone else on matchmaking) today and we completed Arcadia on Heroic but the achievement didn't pop. :\
Willing to give it another go. Also need the Halo Wars 2 achievements:
Two heads are better than one - Completed a mission in co-op
Comrades in arms - Earned a gold medal on any mission (except Prologue) in Co-op on Legendary
Two heads are better than none - Won a Skirmish match with a friend against only AI opponents (2 or more)
If you are interested in trying to knock these 3 out as well that would be awesome.